1. Field of the Invention
The present invention relates to a keyboard assembly, particularly a keyboard assembly suitable for use in an input device such as a word processor or a personal computer.
2. Description of the Related Art
A conventional keyboard assembly used in such an input device is shown in FIGS. 10 and 11. As shown in both figures, a membrane switch is placed on a support plate (not shown), the membrane switch comprising a lower sheet 50 having a fixed contact (not shown) on a lower side thereof and a flexible upper sheet 52 having a movable contact (not shown) and disposed on the lower sheet 50 through an insulating spacer 51.
A support member 53 formed of a resin material is disposed on the membrane switch. The support member 53 is provided with two engaging portions 53a each having an engaging portion 53b and formed on the left-hand side in the figures and is also provided with engaging portions 53c each having an engaging hole 53d and formed on the right-hand side.
A hole 53e is formed between the left- and right-hand side engaging portions 53a, 53c.
A first lever 54 formed of a resin material is provided with a pair of arm portions 54c and a pair of arm portions 54d, each pair having a pair of columnar protrusions 54a and 54b at end portions thereof, with outwardly projecting pivot shaft portions 54e being formed at central positions of the arm portions 54c and 54d. The protrusions 54b are pivotably engaged respectively with the engaging portions 53d of the support member 53.
Outside the first lever 54 is disposed a second lever 55 formed of a resin material. As shown in FIG. 11, the second lever 55 comprises a connecting portion 55a positioned on the right-hand side and a pair of arm portions 55b extending leftwards respectively from upper and lower ends of the connecting portion 55a. The second lever 55 is formed in a generally turned square U-shape.
At both ends of the connecting portion 55a are formed outwardly projecting columnar protrusions 55c respectively, and at the left-hand ends of the paired arm portions 55b are formed outwardly projecting columnar protrusions 55d respectively. A generally elliptic recess 55e is formed centrally in each of the paired arm portions 55b. The protrusions 55d are pivotably engaged respectively with the engaging portions 53a of the support member 53, and the pivot shaft portions 54e of the first lever 54 are fitted in the recesses 55e respectively. In this way the first and second levers 54, 55 are assembled crosswise.
For mounting the first and second levers 54, 55 to the engaging portions 53a and 53c of the support member 53, the first and second levers 54, 55 are held in the crosswise assembled state, and in this state the protrusions 54b are fitted in the engaging holes 53d of the engaging portions 53c while expanding the arm portions 54d of the first lever 54 outwards.
Further, the arm portions 55b of the second lever 55 are narrowed on the side where the protrusions 55d are formed, by applying a force to the arm portions inwards, and the protrusions 55d are allowed to fit in the engaging holes 53b of the engaging portions 53a, whereby the first and second levers 54, 55 can be secured pivotably to the support member 53.
A key top 56 is formed using a resin material, and on the left-hand side of the back of the key top 56 are formed a pair of holding portions 56b in an opposed relation to each other, with slide grooves 56a being formed outwards in the paired holding portions 56b respectively. On the right-hand side of the back of the key top 56 are formed a pair of holding portions 56d, with slide grooves 56c being formed inwards and opposedly to each other in the holding portions 56d respectively.
The protrusions 54a of the first lever 54 are slidably held respectively by the left-hand holding portions 56b, while the protrusions 55c of the second lever 55 are slidably held respectively by the second lever 55, whereby the key top 56 is mounted vertically to be movable on the first and second levers 54, 55.
A domed elastic member 57 formed of an insulating rubber is placed on the membrane switch while being positioned within the hole 53e of the support member 53. An upper end portion of the elastic member 57 is in abutment with the back side of the key top 56.
The first and second levers 54, 55, the key top 56 attached to the levers 54, 55, and the elastic member 57 are arranged in plural rows on the membrane switch to constitute the conventional keyboard assembly.
The conventional keyboard assembly thus constructed operates in the following manner. When the key top 56 is brought down, the first and second levers 54, 55 move pivotally about the protrusions 54b and 55d which are engaged with the engaging portions 53a and 53c formed on the support member 53 side, so that the pivot shaft portions 54e of the first lever 54 move respectively within the generally elliptic recesses 55e of the second lever 55, and the first and second levers 54, 55 pivotally move downward while the protrusions 54a and 55c slide respectively within the slide grooves 56a and 56c formed on the key top 56 side. The key top 56 is adapted to move down with such pivotal downward movement of the first and second levers 54, 55.
As a result of such downward movement of the key top 56, the elastic member 57 is pressed and collapsed and inverted into a dome shape, whereupon the upper sheet, or a switch substrate, 52 is pushed by an inner projection 57a of the elastic member 57 and a circuit contact formed on the switch substrate 52 comes into contact with an electric conductor on the lower sheet, or an insulating substrate, 50 to turn ON the membrane switch.
Upon subsequent relief of the pressure on the key top 56, the first and second levers 54, 55 and the key top 56 are forced back to the respective states before the pressing by the elastic force of the elastic member 57, and the switch substrate 52 which has been pressed down by the elastic member 57 moves out of contact with the insulating substrate 50 and reverts to its initial state by virtue of its own restoring force, so that the membrane switch turns OFF.
In the above conventional keyboard assembly, however, when mounting the first and second levers 54, 55 to the support member 53, the respective protrusions 54b and 55d are attached to the support member 53 while expanding the arm portions 54d of the first lever 54 outwards or narrowing the arm portions 55b of the second lever 55 inwards, thus giving rise to the problem that the assembling work efficiency is deteriorated.